The present invention relates to the improvement of the linearity characteristic of a differential amplifier with respect to an input voltage.
A differential amplifier is used in a large number of electric circuits. The symbol thereof takes such a form as illustrated in FIG. 1 by a character OP. FIG. 2 shows a circuit diagram of one example of the differential amplifier comprised of C-MOS FETs. (Complementary-Metal Oxide Semiconductor Field Effect Transistor). In this example, the MOS transistors M1, M2 and M3 are connected in series between a high level power source voltage V.sub.DD and a low level power source voltage V.sub.SS, thereby constituting a constant voltage source S.sub.cv. The MOS transistors M4 and M5 constitute constant current sources, respectively, and, a constant voltage is applied from the constant voltage source S.sub.cv to the gates thereof. The MOS transistors M6 and M7 constitute a differential operation circuit DOC the gates of which are applied with differential voltages, respectively. The MOS transistors M8 and M9 constitute load elements of the MOS transistors M6 and M7 of the differential operation circuit DOC, respectively. An output voltage Vout of the differential operation circuit DOC is outputted through a MOS transistor M10 constituting an output circuit. To this output circuit, a phase compensation circuit is provided, which phase compensation circuit comprises a series circuit of a capacitor C and a resistor R.
In such a prior art differential amplifier as shown in FIG. 2, however, when the input signal voltage Vin becomes approximate to a low level power source voltage V.sub.SS, the MOS transistors M6 and M7 which constitute the differential operation circuit DOC are cut off with the result that they cease to operate. On the other hand, when the input signal voltage Vin becomes approximate to a high level power source voltage V.sub.DD, the MOS transistors M6 and M7 disadvantageously operate in an unsaturated area. As a result, the linearity of the output signal voltage is impaired. For this reason, in a differential amplifier circuit of so-called "voltage follower connection type" wherein the output terminal is connected to the inverting input terminal (-) as in FIG. 3, the input voltage Vin-output voltage Vout characteristic curve is as graphically shown in FIG. 4. That is to say, as seen in the graphic diagram of FIG. 4, where the input voltage Vin is in the range of V.sub.SS &lt;Vin&lt;V.sub.SS +Va (a range indicated in FIG. 4 by a character "D.sub.L "), the MOS transistors M6 and M7 are cut off with the result that the linearity characteristic of the output voltage Vout is impaired. The voltage Va is determined by the threshold voltage of the MOS transistors M6 and M7, not depending upon the power source voltages. On the other hand, where the input voltage Vin is in the range of V.sub.DD -Vb&lt;Vin&lt;V.sub.DD (a range indicated in FIG. 4 by a character "D.sub.H "), the MOS transistors M6 and M7 also operate in an unsaturated area. In this case as well, the linearity characteristic of the output voltage Vout is impaired. As will be clear from the foregoing, the prior art differential amplifier had a problem in that the range of its input voltage over which the linearity of the output voltage is obtained is narrow. Further, when the high level power source voltage V.sub.DD is set at a low level, the linear range of operation (a range between Va and Vb indicated in FIG. 4 by a notation "D.sub.M ") disadvantageously becomes narrow. For this reason, a reduction in the power source voltage V.sub.DD has hitherto been hindered. Further, when the input voltage Vin is displaced from an operating point of the differential amplifier, the output voltage is distorted.